The present invention relates to an arrangement for generating an SSB signal and has applications in both the analogue and conventional digital implementations single sideband modulators constructed in accordance with what is termed Weaver's method. Such modulators are used in SSB generation in speech and data transmission for mobile radios.
Weaver's method is disclosed in Proceedings of the IRE "A Third Method of Generation and Detection of Single-Sideband Signals" Donald K. Weaver, Jr., pages 1703 to 1705. FIG. 1 of the accompanying drawings illustrates a modulator made in accordance with Weaver's method. An analogue input signal at an input 10 is band limited in a bandpass filter 12 and then is applied to balanced mixers 14, 16 where it is mixed with quadrature related carrier signals cos (.omega..sub.o t) and sin (.omega..sub.o t), where .omega..sub.o t is a carrier frequency corresponding to the centre of the passband of the signal from the filter 12. As a result of these mixing operations quadrature related signals I and Q are produced which have single sideband components at base band and unwanted components at high frequencies. The wanted components of the I and Q signals are selected by low pass filters 18, 20 and then frequency up-converted to their carrier frequency (.omega..sub.c t) in balanced mixers 22, 24. The signals from these mixers are combined in a summing circuit 25 to provide an output on a terminal 26.
In data transmission a similar arrangement to that shown in FIG. 1 is used to frequency up-convert the baseband I and Q modulated signals corresponding to the data. Weaver's method may also be used to generate constant envelope signals suitable for mobile radio digital transmission.
A drawback to Weaver's method is that it is difficult to implement the frequency up-converting or RF section as an integrated circuit. Reasons for this difficulty include the fact that matching of the mixers 22, 24 (FIG. 1) is complicated and it may be necessary to run the mixers at a high power level to achieve the desired performance. Also it is necessary to make a pair of quadrature local oscillators capable of generating a wide range of frequencies. Any mismatch in the mixers or any imperfection in the quadrature RF signal generation may cause the unwanted signal levels to increase.
Mismatch problems mentioned above can be overcome by generating digital samples of the RF local oscillators and using digital multipliers instead of the RF mixers. This is shown in FIG. 2 of the accompanying drawings which illustrate a conventional digital implementation of Weaver's method. In FIG. 2 the output of the bandpass filter 12 is digitised in an analogue-to-digital converter 28 which is sampled at a clock frequency f.sub.s1. The digitised signals are applied to mixers 14, 16 which respectively receive digitised local oscillator signals cos [(2.pi.f.sub.1.(m/f.sub.s1)] and sin (2.pi.f.sub.1.m/f.sub.s1). The I and Q signals from the mixers are low pass filtered in digital filters 18, 20 which are clocked at the frequency f.sub.s1 which satisfies the Nyquist sampling rate. The output signals which have been sampled at f.sub.s1 are applied to respective interpolators 30, 32 operating at a sampling frequency N.sub.fs. The outputs of the interpolators 30, 32 are frequency up-converted in digital multipliers 34, 36 to which are supplied quadrature carrier signals cos [2.pi.f.sub.c.(n/Nf.sub.s)] and sin [2.pi.f.sub.c.(n/Nf.sub.s)], respectively. The outputs from the digital multipliers 34, 36 are combined in the summing circuit 25 and the R.F. output appears on the terminal 26. This digital implementation of Weaver's method enables a performance improvement to be achieved at the expense of increased power consumption as digital multipliers require more power than the equivalent analogue mixers. The generation of RF quadrature signal samples raises another problem for radio transmitter integration when constant frequency sampling is required at RF. The sine and cosine samples can be generated by using either look-up tables or on-line processing but neither method is attractive for integration.